The food industry uses numerous bacteria, in particular lactic acid bacteria, in order to improve the taste and the texture of foods but also in order to extend the shelf life of these foods. In the case of the dairy industry, lactic acid bacteria are used intensively in order to bring about the acidification of milk (by fermentation) but also in order to texturize the product into which they are incorporated.
Among the lactic acid bacteria used in the food industry, there can be mentioned the genera Streptococcus, Lactococcus, Lactobacillus, Leuconostoc, Pediococcus and Bifidobacterium. The lactic acid bacteria of the species Streptococcus thermophilus are used extensively alone or in combination with other bacteria such as Lactobacillus delbrueckii subsp bulgaricus for the production of food products, in particular fermented products. They are used in particular in the formulation of the ferments used for the production of fermented milks, for example yoghurts. Certain of them play a dominant role in the development of the texture of the fermented product. This characteristic is closely linked to the production of polysaccharides.
The current trend in yoghurts is for mild flavor and high texture. Today this is achieved by the use of cultures which produce a mild flavor and the addition of thickeners or protein to give the desired thickness. Yoghurt producers would like to be able to make yoghurt with these properties without the addition of thickening agents. This will help them reduce cost and give a cleaner label. One very attractive way to achieve this would be to have a starter culture which produces a high level of texture.
In order to meet the requirements of the industry, it has become necessary to provide novel texturizing strains of lactic acid bacteria, in particular of Lactobacillus delbrueckii subsp bulgaricus and Streptococcus thermophilus, for texturizing food products. Especially there is a need for novel texturizing strains of Lactobacillus delbrueckii subsp bulgaricus which can be used together with novel texturizing strains of Streptococcus thermophilus. 
Mutants resistant towards D-cycloserine have been described for a number of different bacteria but it has never been reported that these mutants can be used for increasing the texture of a dairy product. Neither Streptococcus thermophilus nor Lactobacillus delbrueckii subsp bulgaricus mutants have been described.